Archaeologists have uncovered ancient bones that may rewrite American history

San Diego Natural History Museum Paleontologist Don Swanson pointing at a rock fragment near a piece of mastodon tusk.
San Diego Natural History Museum
The first humans to set foot in the Americas arrived some 25,000 years ago — or so we thought.

For decades, that date has been generally accepted by scientists, though recent genetic studies have moved the dial on that figure back by a few hundred or thousand years. But a set of new, highly controversial evidence suggests that timeline could be fundamentally incorrect.

Researchers working at an archaeological dig site that runs along the 54 freeway in San Diego, California, have uncovered what they believe is evidence of a human presence in North America that predates previous estimates by 100,000 years. They published their findings Wednesday in a paper in the well-regarded scientific journal Nature.

"If this is true," Mikkel Winther Pedersen, a geogeneticist at the University of Copenhagen in Denmark who was not directly involved in the study, tells Business Insider, "it would rock the ground that we are standing on at the moment, not just for all archaeologists but for all the other researchers interested in this."

The proposed timeline revision is based on a set of 130,000-year old mastodon bones (dated using uranium) that show signs of having been processed by humans, according to the paper. At the archaeological site, which was first unearthed in the 1990s, researchers discovered pieces of limb bones and teeth from the mastodon, an enormous extinct creature distantly related to the elephant.

The archaeologists say the way those bones were broken tells an important story.

San Diego Natural History Museum
Over hundreds of thousands of years, the ground beneath us gives way to tiny seismic shifts. These push-and-pull forces crush bone, hack apart human and animal remains, and turn solids to dust. But instead of just showing the typical patterns of decay that bones exhibit over time, many of the fragments appeared to have been fractured shortly after the animal died. This is important because it signals that something other than natural processes were at work.

"At many sites you have evidence that bones were used for hammers or anvils," says Richard Fullagar, an archaeologist at Australia's University of Wollongong. "What's truly remarkable at this site is you can identify a particular hammer that was hit on a particular anvil."

Together, all of this data paints a picture that Fullagar calls "incontrovertible" evidence that humans were around at the time this mastodon died.

The surface of a mastodon bone showing a notch the researchers believe was created by humans.
Tom Deméré, San Diego Natural History Museum

"It's really the age of the site that's the extraordinary part of this research," Thomas Demere, a paleontologist at the San Diego Natural History Museum, said on a call with reporters on Tuesday. "It makes ours the oldest site in the Americas by a factor of 10."

Not everyone agrees that the evidence points to a human presence, however.

Michael Waters, director of the Center for the Study of the First Americans at Texas A&M University, tells Business Insider via email that the bones found at the site, while intriguing, don't prove that humans were ever there.